Using an insolubilized derivative of prostaglandin E1 we have recently shown that platelet poor plasma previously exposed to the insolubilized hormone is capable of inhibiting ADP or 1-epinephrine induced platelet aggregation (Science 200, 202, 1978). The results indicated the existence of a cyclic AMP independent pathway for the inhibition of platelet aggregation. The objectives of the proposed research are: (1) To identify and purify the plasma factor(s) which mediates the inhibition of platelet aggregation induced by the hormone. Identification will be attempted using plasmas deficient in factors involved in blood coagulation and platelet aggregation. In addition, normal plasma will be fractionated by gel filtration, ion-exchange chromatography and affinity techniques. (2). To understand the mechanism of the hormone-induced but cyclic AMP independent inhibition of platelet aggregation, the effects of plasma exposed to insolubilized prostaglandin E1 on Ca ions uptake and cyclic AMP independent protein kinase in platelets will be assessed. The possibility that cyclic AMP might be involved in the synthesis and release of prostaglandin E1 in platelets will also be tested. These will be determined by the combination of thin layer chromatography and radioimmunoassay.